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Quantum-effect Nanoscale Devices

According to the laws of quantum mechanics, free carriers in a metal or semiconductor can only take on specific values of energy, as defined by the crystal structure; that is, the energy is quantized. For most practical purposes, there are so many closely spaced energy levels, it appears that the carriers have a continuum of possible energies, except for the well-defined gaps characteristic of semiconductors. When the carrier is confined to a region where one or more of the dimensions reach the range of less than 100 nm, the quantum energy levels begin to spread out and the quantum nature becomes detectable. Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. This reduction in size can take place in one, two, or three dimensions, using the fabrication techniques discussed earlier, yielding structures known respectively as superlattices, quantum wires, and quantum dots.

When electrons are introduced into a semiconductor structure, they migrate to those positions where their energy is lowest, much like a ping-pong ball will come to rest in a dimple on a waffled surface. If the nanostructure is engineered correctly, then the electrons will settle in the nanostructure itself and not in the adjacent layers. These carriers will then exhibit the quantum effects imposed on them by the nanostructure. The ability to engineer artificial atoms and molecules in semiconductors using nanofabrication techniques has resulted in a powerful new tool in creating novel semiconductor devices, such as quantum dots where the number of carriers trapped by the dot can be controlled by an external voltage.

It appears possible that nanoscale quantum-effect devices may become widely used in complex electronic systems, such as a neural array of quantum dots spaced only a few 100 nm apart, but this will only take place after significant progress has been made in fabrication and tolerance.

Scientists see a universe of potential in nanotechnology, following years or perhaps decades of research and development. Some of the applications they foresee are as follows: surgical instruments of molecular scale that are guided by computers of the same size; rocket ships for the individual made of shatterproof materials created by nanomachines; synthesis of foods by molecules and an end to famine; pollution-free manufacturing and molecular devices that clean up existing pollution without human intervention; consumer goods that assemble themselves; reforming of soil (termed terraforming) both on Earth and other planets where soil and rock may not exist; and computers capable of more computations in 1 second than all the existing semiconductor devices in the world combined. Nanodevices may create "smart mat ter" that, when used to build a bridge or a high-rise building, knows when and how to repair itself; diamonds of perfect quality and any size may be built atom by atom to suit industrial needs or an individual's ideal; injectable molecular robots that enter the bloodstream on seek-and-destroy missions for cancer, AIDS, invading bacteria or viruses, and arterial blockages. Similarly, nanoparticles might carry vaccines and drugs directly to the source of the ailment.

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Science EncyclopediaScience & Philosophy: Mysticism to Nicotinamide adenine dinucleotideNanotechnology - Nanofabrication Techniques, Theoretical Methods, Conventional Nanoscale Devices, Quantum-effect Nanoscale Devices, Tangible Advances